How to access character (physics type) via python to update it's jump force in real time?
Current api (2.78c) GameObjectSettings.jump_speed

GameObjectSettings is part of bpy which only work inside of blender and only in Blender, this means that your game using bpy and bge only will work inside Blender because bpy will not work in standalone and you dont want that if you are making a game but if you are makin sortsof simulation and you will be using always blender then you can use bge and bpy.

You can access jump_speed:

from bpy import data

data.objects["Cube"].game.jump_speed = 50
print(data.objects["Cube"].game.jump_speed)


As soon as you run the script 50 will be the value for jump force in physics property panel:



here you can found some (old) good bge python tutorials

If you want in game dinamically change the jump speed you can use the next script portion was extracted from one of the tutorials, thanks to Goran Milovanovic

script

from bge import logic
from mathutils import Vector
import math

class MoveWSAD:
    """
    Goran's 2.6x FPS base-frame

    2011 - Goran Milovanvic - nilunder.com

    """
    def __init__(self, cont, own):
        self.cont = cont
        self.body = own
        self.sen_key_w = self.cont.sensors["W"]
        self.sen_key_s = self.cont.sensors["S"]
        self.sen_key_a = self.cont.sensors["A"]
        self.sen_key_d = self.cont.sensors["D"]
        self.sen_key_space = self.cont.sensors["space"]
        self.sen_col_onground = self.cont.sensors["onground"]
        self.speed_ground = 4
        self.speed_jump = self.speed_ground * 0.8
        self.main = self._inAir

    def _getMoveVec(self):
        forward_back = self.sen_key_w.positive - self.sen_key_s.positive
        left_right = self.sen_key_d.positive - self.sen_key_a.positive
        velocity = Vector((left_right, forward_back, 0))
        return velocity

    def _onGround(self):
        vec_velocity = self._getMoveVec()
        vec_velocity.magnitude = self.speed_ground
        if self.sen_key_space.positive:
            vec_velocity.z = self.speed_jump
        self.body.setLinearVelocity(vec_velocity, 1)
        if not self.sen_col_onground.positive:
            self.main = self._inAir

    def _inAir(self):
        if self.sen_col_onground.positive:
            self.main = self._onGround

"""Strapicarus 2017"""                
class Player:       
    def __init__(self, cont, body):       
        self.body = body
        self.move_wsad = MoveWSAD(cont, body)
        self.main = self.walking

    def walking(self):
        if self.move_wsad.sen_key_space.positive:
             self.change_speed(1)
        self.move_wsad.main()


    def change_speed(self, speed):
        if self.move_wsad.speed_jump &lt; 20:
            self.move_wsad.speed_jump += speed
        else:
            self.move_wsad.speed_jump = self.move_wsad.speed_ground * 0.8
        return

def main(cont):
    if not 'init' in cont.owner.getPropertyNames():
        cont.owner['player'] = Player(cont, cont.owner)
        cont.owner['init'] = True
    cont.owner['player'].main()

main(logic.getCurrentController())


LogicBricks



Physics panel



EDIT ADDED

Well a just download your blend and you put the right blend this time (i forgot to mention that before answer), and i test 
¿if i change the loc z in motion actuator the character will jump higher? and YES it works.

So you just need increase the loc z value to set the jump speed for that motion actuator:



And if you want a python way here is to:

jump.py

from bge import logic

def main(cont):
    jumpact = cont.actuators['jumpSpeed']
    colsen = cont.sensors['Blue']
    if not 'init' in cont.owner.getPropertyNames():
        cont.owner['init'] = True
        cont.owner['_onair'] = False
        cont.owner['jumpSpeed'] = 0.0
        print('init...')
        return   

    if not cont.owner['_onair'] and colsen.positive:
        if  cont.owner['jumpSpeed'] &lt; 10.0:
            cont.owner['jumpSpeed'] += 1 
            jumpact.dLoc = (0,0,cont.owner['jumpSpeed'])
            print('adding jump speed')
            print(jumpact.dLoc.z)
        else:
            cont.owner['jumpSpeed'] = 0.0
        cont.activate(jumpact)
        cont.owner['_onair'] = True
        return
    else:
        cont.deactivate(jumpact)
        cont.owner['_onair'] = False
        return
#    print(dir(jumpact))
#    print(jumpact.force)
#    print(jumpact.linV)
#    print(jumpact.torque)
#    print(jumpact.dLoc) #FOUND IT ! xD

main(logic.getCurrentController())


Logic editor



Blend file